The invention relates to a laser diode module comprising:
a rectangular box-shaped metal casing with a bottom, two long and two short side walls, a tubular passage for a glass fiber being provided in one of the short side walls, and at least six guide pins being passed through the bottom in a standardized DIL order, at least one of the guide pins being inserted in the bottom by means of electrically conductive material and the remaining guide pins being inserted in the bottom by means of feedthrough insulators,
a sheet-like metal base carrier having a electrically conductive coupling with the casing, on which base carrier are mounted: a first subcarrier with a laser diode attached to it, a support for mounting the glass fiber in a manner such that its end faces the front facet of the laser diode and a second subcarrier with the photodiode attached to it, and
connecting lines for connecting the laser diode, the photodiode and the metal base carrier to the respective guide pins.
In optical communication systems laser diode modules are used for electro-optical conversion of the electric data signals to be transferred, whereas systems with data bit rates of 140, 280 and 565 Mbit/s are already widely used. In the 565 Mbit/s systems a laser diode module having the configuration mentioned in the preamble has become a de facto standard.
In the field of optical communication there is a tendency towards bit rates increasing in the short term, according to which tendency systems having a bit rate of 2.4 Gbit/s are already being developed. With these increasing bit rates it appears that from about 1 Gbit/s not so much the laser diode itself, but rather the laser diode module as a whole forms the constraining factor. Therefore, every attention should be given to the electrical features of the complete laser diode module, specifically, its small-signal response and reflection coefficient.
In the transmitter of such a communication system a laser driving circuit is included which is connected to a laser diode module through an external transmission line. In this context a proper adjustment to the characteristic impedance of the transmission line is desirable at the two ends of this transmission line, because the lack of a proper adjustment will result in signal reflections between the driving circuit and the laser diode module which may lead to disturbances of the pulse shape of the data signals applied to the laser diode. Since the laser diode itself has a relatively low impedance, usually a resistor is inserted between the external transmission line and the signal input to the laser diode module, the resistance of this resistor being such that the impedance of the series connection of this resistor and the laser diode matches the characteristic impedance of the transmission line as much as possible.
For low bit rates of the data signals the adaptation by means of a resistor appears to be rather satisfactory. However, at increased bit rates the relatively large length of the connecting wires to the guide pins and the length of the guide pins themselves start playing an ever more important role in the laser diode module and the matching appears to degrade increasingly. The consequent reflection phenomena appear to augment as a function of frequency and hence the frequency range in which the laser diode module can be used with acceptable disturbances of the pulse shape of the data signals applied to the laser diode is limited.
A possible solution to achieve a proper adjustment to the characteristic impedance of the transmission line is completely redesigning the laser diode module for bit rates exceeding 1 Gbit/s. For this design the dimensioning of the casing and the order of guide pins fed through the casing are to be modified such that the electrical characteristics for higher frequencies are more favorable than with the standard module. This solution is not only costly, but is also accompanied with the disadvantage that the printed circuit board comprising the electric circuit elements to be connected to the laser diode module is to be re-designed, because the connecting pins for the module guide pins can in that case no longer be chosen according to a standardized DIL order.